With the recent spread of copiers and printers which are based on electrophotography, accompanying demands have been made that these copiers and printers be energy-saving (diminished power consumption), particularly in the are of domestic use, possess an increasing number of functions, and be capable of being operated at a higher speed particularly for the so-called gray area located between printing machines and copiers. There also is demand for copiers or printers which can be operated at a lower rolling pressure to thereby simplify the fixing-roll and reduce machine cost. In addition, since copiers that have a double-side-copying function or are equipped with an automatic document feeder have spread widely, due to the trend toward shifting to higher-grade copiers, the electrophotographic toners employed in such copiers and printers are required to have a low fixing temperature, to be less apt to cause offset, and to display excellent affixing strength to receiving paper so as to avoid smearing during both-side copying and automatic document feeder operations.
To meet the requirements described above, the following prior art techniques, which involve utilizing a binder resin having an improved molecular weight or improved molecular weight distribution, have been proposed.
Specifically, an attempt has been made to employ a binder resin having a reduced molecular weight to thereby attain a lower fixing temperature. However, the reduction in molecular weight has also resulted in a reduced viscosity and a lower melting point. Consequently, offset to the fixing roll may result. To avoid this offset phenomenon, a technique of modifying the molecular weight distribution of the binder resin by widening the low-molecular-weight region and high-molecular-weight region thereof and a technique of crosslinking a high-molecular part of the binder resin have been employed. However, use of these techniques necessitates a reduction in the glass transition temperature of the resin so as to impart sufficient fixability to the toner, which unavoidably impairs the storage stability of the toner formed.